Yeah Unity's a good shout. They have a section on the Asset Store of free assets they recommend for GGJ, Looks like some of them are just projects from their tutorials, but there's probably loads of useful stuff in there. https://www.assetstore.unity3d.com/en/#!/list/34942-global-game-jam-starter-kit

Yes, Unity is always very popular! Often, it depends on what tools/languages you're most comfortable using already, but if you're looking for ideas for things to try ahead of time check out Haxe-Flixel, Twine and Inform7 too :)